Most polycrystalline materials have within their elastic range an almost constant relationship between stress and strain. Experiments by an English scientist named Robert Hooke led to the formation of Hooke's Law, which states that in the elastic range of a material strain is proportional to stress. The ratio of stress to strain, or the gradient of the stress-strain graph, is called the Young's Modulus.

EO 1.10 DEFINE the following terms:

a. Bulk Modulus

b. Fracture point

EO 1.11 Given stress-strain curves for ductile and brittle material, IDENTIFY the following specific points on a stress-strain curve.

a. Proportional limit c. Ultimate strength

b. Yield point d. Fracture point

EO 1.12 Given a stress-strain curve, IDENTIFY whether the type of material is ductile or brittle.

EO 1.13 Given a stress-strain curve, INTERPRET a stress-strain curve for the following:

a. Application of Hooke's Law

b. Elastic region

c. Plastic region

Elastic Moduli

The elastic moduli relevant to polycrystalline material are Young's Modulus of Elasticity, the Shear Modulus of Elasticity, and the Bulk Modulus of Elasticity.

Young's Modulus

Young's Modulus of Elasticity is the elastic modulus for tensile and compressive stress and is usually assessed by tensile tests. Young's Modulus of Elasticity is discussed in detail in the preceding chapter.

Shear Modulus

The Shear Modulus of Elasticity is derived from the torsion of a cylindrical test piece. Its symbol is G.

Bulk Modulus

The Bulk Modulus of Elasticity is the elastic response to hydrostatic pressure and equilateral tension or the volumetric response to hydrostatic pressure and equilateral tension. It is also the property of a material that determines the elastic response to the application of stress.